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Held for Sale and Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Sep. 28, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 28, 2017
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Earned insurance premiums |  | $ 3.3 | $ 2.4 | $ 9.7 | $ 6.6 |
Net investment income |  | 13.4 | 12.2 | 44.9 | 39.7 |
Net realized and unrealized investment (losses) gains |  | 66.1 | 32.5 | 22.9 | 102.5 |
Total revenues |  | 198.7 | 87.5 | 363.1 | 259.8 |
Insurance acquisition expenses |  | 1.2 | 0.9 | 3.9 | 3.0 |
Other underwriting expenses |  | 0.1 | 0.1 | 0.3 | 0.3 |
General and administrative and other expenses |  | 68.5 | 41.7 | 174.6 | 156.1 |
Interest expense |  | 3.4 | 0.9 | 6.0 | 1.8 |
Total expenses |  | 154.4 | 79.1 | 397.2 | 257.2 |
Net (loss) income from discontinued operations, net of tax |  | 0.0 | (15.2) | 0.0 | 20.5 |
Net (loss) gain from sale of discontinued operations, net of tax |  |  | 554.3 | (17.2) | 552.7 |
Total income (loss) from discontinued operations |  | (17.3) | 539.1 | (17.2) | 573.2 |
Total comprehensive income (loss) |  | 40.0 | 565.1 | (5.4) | 608.1 |
OneBeacon |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Net (loss) income from discontinued operations, net of tax |  |  | (15.2) |  |  | $ 20.5
Net (loss) gain from sale of discontinued operations, net of tax | $ 554.6 |  |  |  |  |
Other Discontinued Operations |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Total comprehensive income (loss) |  |  |  |  | (1.0) |
Discontinued Operations |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Total income (loss) from discontinued operations |  | (17.3) | 539.1 | (17.2) | 573.2 |
Discontinued Operations | Net gain (loss) from sale of discontinued operations |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Net (loss) gain from sale of discontinued operations, net of tax |  |  | 554.5 |  | 554.5 |
Discontinued Operations | OneBeacon |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Earned insurance premiums |  |  | 268.4 |  | 807.6 |
Net investment income |  |  | 13.0 |  | 39.7 |
Net realized and unrealized investment (losses) gains |  |  | 11.5 |  | 38.8 |
Other revenue |  |  | 2.2 |  | 7.7 |
Total revenues |  |  | 295.1 |  | 893.8 |
Loss and loss adjustment expenses |  |  | 206.8 |  | 546.0 |
Insurance acquisition expenses |  |  | 51.9 |  | 145.6 |
Other underwriting expenses |  |  | 44.9 |  | 156.2 |
General and administrative and other expenses |  |  | 7.4 |  | 21.2 |
Interest expense |  |  | 3.4 |  | 10.0 |
Total expenses |  |  | 314.4 |  | 879.0 |
Pre-tax (loss) income |  |  | (19.3) |  | 14.8 |
Income tax benefit |  |  | 4.1 |  | 5.7 |
Net (loss) income from discontinued operations, net of tax |  |  | (15.2) |  | 20.5 |
Total income (loss) from discontinued operations |  |  | 539.3 |  | 575.0 |
Net change in foreign currency translation and other |  |  | 0.1 |  | 0.3 |
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax |  |  | 2.9 |  | 2.9 |
Total comprehensive income (loss) |  |  | 542.3 |  | 578.2 |
Discontinued Operations | Sirius Group |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Earned insurance premiums |  |  | 0.0 |  | 0.0 |
Net investment income |  |  | 0.0 |  | 0.0 |
Net realized and unrealized investment (losses) gains |  |  | 0.0 |  | 0.0 |
Other revenue |  |  | 0.0 |  | 0.0 |
Total revenues |  |  | 0.0 |  | 0.0 |
Loss and loss adjustment expenses |  |  | 0.0 |  | 0.0 |
Insurance acquisition expenses |  |  | 0.0 |  | 0.0 |
Other underwriting expenses |  |  | 0.0 |  | 0.0 |
General and administrative and other expenses |  |  | 0.0 |  | 0.0 |
Interest expense |  |  | 0.0 |  | 0.0 |
Total expenses |  |  | 0.0 |  | 0.0 |
Pre-tax (loss) income |  |  | 0.0 |  | 0.0 |
Income tax benefit |  |  | 0.0 |  | 0.0 |
Net (loss) income from discontinued operations, net of tax |  |  | 0.0 |  | 0.0 |
Net (loss) gain from sale of discontinued operations, net of tax |  | $ (17.3) | (0.2) | $ 0.1 | (0.8) |
Total income (loss) from discontinued operations |  |  | (0.2) |  | (0.8) |
Total comprehensive income (loss) |  |  | (0.2) |  | (0.8) |
Discontinued Operations | Sirius Group | Net gain (loss) from sale of discontinued operations |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Net (loss) gain from sale of discontinued operations, net of tax |  |  | (0.2) |  | (0.8) |
Discontinued Operations | Other Discontinued Operations |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Earned insurance premiums |  |  |  |  | 0.0 |
Net investment income |  |  |  |  | 0.0 |
Net realized and unrealized investment (losses) gains |  |  |  |  | 0.0 |
Other revenue |  |  |  |  | 0.0 |
Total revenues |  |  |  |  | 0.0 |
Loss and loss adjustment expenses |  |  |  |  | 0.0 |
Insurance acquisition expenses |  |  |  |  | 0.0 |
Other underwriting expenses |  |  |  |  | 0.0 |
General and administrative and other expenses |  |  |  |  | 0.0 |
Interest expense |  |  |  |  | 0.0 |
Total expenses |  |  |  |  | 0.0 |
Pre-tax (loss) income |  |  |  |  | 0.0 |
Income tax benefit |  |  |  |  | 0.0 |
Net (loss) income from discontinued operations, net of tax |  |  |  |  | 0.0 |
Total income (loss) from discontinued operations |  |  |  |  | (1.0) |
Discontinued Operations | Other Discontinued Operations | Net gain (loss) from sale of discontinued operations |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Net (loss) gain from sale of discontinued operations, net of tax |  |  |  |  | (1.0) |
Discontinued Operations | Total |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Earned insurance premiums |  |  | 268.4 |  | 807.6 |
Net investment income |  |  | 13.0 |  | 39.7 |
Net realized and unrealized investment (losses) gains |  |  | 11.5 |  | 38.8 |
Other revenue |  |  | 2.2 |  | 7.7 |
Total revenues |  |  | 295.1 |  | 893.8 |
Loss and loss adjustment expenses |  |  | 206.8 |  | 546.0 |
Insurance acquisition expenses |  |  | 51.9 |  | 145.6 |
Other underwriting expenses |  |  | 44.9 |  | 156.2 |
General and administrative and other expenses |  |  | 7.4 |  | 21.2 |
Interest expense |  |  | 3.4 |  | 10.0 |
Total expenses |  |  | 314.4 |  | 879.0 |
Pre-tax (loss) income |  |  | (19.3) |  | 14.8 |
Income tax benefit |  |  | 4.1 |  | 5.7 |
Net (loss) income from discontinued operations, net of tax |  |  | (15.2) |  | 20.5 |
Total income (loss) from discontinued operations |  |  | 539.1 |  | 573.2 |
Net change in foreign currency translation and other |  |  | 0.1 |  | 0.3 |
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax |  |  | 2.9 |  | 2.9 |
Total comprehensive income (loss) |  |  | 542.1 |  | 576.4 |
Discontinued Operations | Total | Net gain (loss) from sale of discontinued operations |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Net (loss) gain from sale of discontinued operations, net of tax |  |  | $ 554.3 |  | $ 552.7 |

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